Communications systems employing radio-frequency (RF) transmitters and receivers are very common and well-known. One application of such communication systems is in the field of automatic, body-implantable medical devices, such as pacemakers, defibrillators, neural stimulators, and the like. RF communication is used to establish "downlink" telemetry channels, in which operational data and commands are transmitted from an external programming unit transmitter to a receiver in an implanted device, and to establish "uplink" telemetry channels, in which information is transmitted from the implanted device's transmitter to a receiver in the external unit.
A specific example of a telemetry system for body-implantable medical devices is the Medtronic Model 9790 Programmer, commercially available from the assignee of the present invention. The Model 9790 Programmer, with appropriate software modules, can be used to communicate (both uplink and downlink) with numerous body-implantable devices manufactured by Medtronic, Inc.
In the prior art, as typified by the Model 9790 Programmer in conjunction with a Medtronic implantable pacemaker, for example, an antenna in the form of a multiple-turn wire coil is disposed within the hermetic enclosure of the implanted device. Downlink RF signals transmitted to the implanted device induce a current in the coil antenna, and this current is amplified and applied to a receiver input for demodulation and extraction of the information content of the RF signal. Similarly, for uplink communication, electrical currents applied directly to the implanted coil antenna cause RF electromagnetic signals-to be generated, such that these signals can be received by a corresponding antenna associated with the external unit.
For various reasons, including the desire to minimize the necessary strength of both uplink and downlink telemetry signals in implantable device systems, the external unit of an implantable device system typically includes a relatively small, handheld programming head containing the external antenna, so that this programming head can be placed directly over the implant site of the implanted device, thus minimizing the distance between the implanted and external antennas. The head is typically connected to the larger base unit of the programmer via a multiple-conductor cable. The aforementioned Model 9790 is one example of a implantable device programmer having this configuration. The Model 9790 is described in further detail in co-pending U.S. Pat. No. 5,345,362 to Winkler, entitled "Portable Computer Apparatus With Articulating Display Panel." The Winkler '362 patent is hereby incorporated by reference herein in its entirety.
As will be appreciated by those of ordinary skill in the art, an important principle underlying RF communication systems is that time-varying electric fields ("E-fields") produce magnetic fields ("H-fields"), while time-varying magnetic fields produce electric fields. Those of ordinary skill in the art will also appreciate that RF signals such as are transmitted and received in RF communication systems have two components: an electric field component and a magnetic field component. The E-field and H-field components of an RF signal are interrelated but distinct (as expressed by Maxwell's equations).
In many communication system applications, including typical telemetry circuits for implantable medical device systems, it is the H-field component that is of interest, rather than the E-field component, since it is the time-varying H-field component of an RF signal which is capable of inducing a current in a coil antenna (i.e., which is capable of being "received"). As a result, the E-field component of RF communication signals in many cases is regarded as "noise" to be ignored. Accordingly, telemetry systems are often provided with shielding which attenuates or shields E-fields, but which is permeable to H-fields.
Magnetic-permeable E-shields can be implemented in a number of ways. One common implementation involves surrounding components to be protected from unwanted E-fields with a metallic shroud or cage. Magnetic permeability is realized by ensuring that the metallic enclosure is "broken" or non-continuous (i.e., it must have gaps), so that it does not provide a short-circuit for impinging H-fields and thus shield both E-fields and H-fields.
Although a discontinuous metallic shroud in electro-magnetic equipment is a fairly straightforward and effective E-shield, there are several potential difficulties with such an arrangement in practical implementation, particularly in application in programming heads for implantable medical devices. One such difficulty arises simply from the fact that one or more extra mechanical components--the metallic shroud itself--are required. ( Usually, because the shield must be discontinuous, the shield comprises at least two parts.) This requirement increases the complexity of the design and assembly, and perhaps the size of the device. Additionally, the need to electrically couple the metallic shield to a ground terminal in the programming head, and ultimately to a ground conductor in the cable connecting the head to the programmer base unit, increases the amount of hardware necessary, increases the complexity of the design and assembly of the device, and increases the risk of electrical or mechanical failure of the programming head, e.g., due to mishandling, dropping, age, etc. . . .
A further difficulty in providing a metallic E-shield is that it is typically desirable to dispose E-shielding as physically far from the antenna as possible, to avoid capacitive coupling of the shield and antenna.